The subject matter disclosed herein relates to imaging of biological tissues using fluorescence from intrinsic and/or extrinsic agents. One of today's main challenges of the biomedical optical community is to image deeper into a layer of biological tissue. Choosing the appropriate imaging wavelength according to light attenuation caused by tissue provides a means of increasing the imaging depth. There are “optical windows” (also known as therapeutic windows) for biological tissues in the far-red to near infrared (NIR) range which allows light to penetrate deep into tissue. In the ultraviolet to visible, the limitation of imaging depth in tissue is due to the scattering owing to extracellular and nuclear structures and attenuation of blood, e.g. oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) and water. There is a first optical window between about 650 nm and about 1100 nm. There is a second window between about 1200 nm and about 1350 nm. There is a third optical window between about 1550 nm to 1800 nm. In the second and third optical windows scattering is reduced but absorption is larger. The diffusive component is reduced in the second and third windows.
The penetration depth of current imaging techniques are still limited by the scattering of the operating light in the visible range and re-absorption of the emitted light. For instance the dye fluorescein, excited by a Ti:sapphire laser beam at 800 nm, emits at 521 nm which is in the visible range. Additionally, visible intrinsic fluorescence such as tryptophan, collagen, elastin, flavins and NADH restricts molecular imaging with exogenous contrast agents, particularly when target concentrations are low emit below 600 nm from 340 nm to 520 nm.
It would be desirable to provide alternative methods of optically imaging biological tissues at a greater depth. To date, no method has been entirely satisfactory. The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.